For purposes of clarity, the term “conventional engine” as used in the present application refers to an internal combustion engine wherein all four strokes of the well known Otto cycle (i.e., the intake, compression, expansion and exhaust strokes) are contained in each piston/cylinder combination of the engine. The term split-cycle engine as used in the present application may not have yet received a fixed meaning commonly known to those skilled in the engine art. Accordingly, for purposes of clarity, the following definition is offered for the term “split-cycle engine” as may be applied to engines disclosed in the prior art and as referred to in the present application.
A split-cycle engine as referred to herein comprises:
a crankshaft rotatable about a crankshaft axis;
a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft;
an expansion (power) piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft; and
a crossover passage interconnecting the expansion and compression cylinders, the crossover passage including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween.
U.S. Pat. No. 6,543,225 granted Apr. 8, 2003 to Carmelo J. Scuderi contains an extensive discussion of split-cycle and similar type engines. In addition the patent discloses details of a prior version of an engine of which the present invention comprises a further development.
Referring to FIG. 1, an exemplary embodiment of the prior art split-cycle engine concept is shown generally by numeral 10. The split-cycle engine 10 replaces two adjacent cylinders of a conventional four-stroke engine with a combination of one compression cylinder 12 and one expansion cylinder 14. These two cylinders 12, 14 perform their respective functions once per crankshaft 16 revolution. The intake charge (fuel and air) is drawn into the compression cylinder 12 through typical poppet-style intake valves 18. The compression cylinder piston 20 pressurizes the charge and drives the charge through the crossover passage 22, which acts as the intake passage for the expansion cylinder 14.
A check type crossover compression (XovrC) valve 24 at the crossover passage inlet is used to prevent reverse flow from the crossover passage 22. A crossover expansion (XoveE) valve 26 at the outlet of the crossover passage 22 controls flow of the pressurized intake charge into the expansion cylinder 14. Spark plug 28 is fired soon after the intake charge enters the expansion cylinder 14 and the resulting combustion drives the expansion cylinder piston 30 down. Exhaust gases are pumped out of the expansion cylinder through poppet exhaust valves 32.
With the split-cycle engine concept, the geometric engine parameters (i.e., bore, stroke, connecting rod length, compression ratio, etc.) of the compression and expansion cylinders are generally independent from one another. For example, the crank throws 34, 36 for each cylinder may have different radii and be phased apart from one another with top dead center (TDC) of the expansion cylinder piston 30 occurring prior to TDC of the compression cylinder piston 20. This independence enables the split-cycle engine to potentially achieve higher efficiency levels and greater torques than typical four stroke engines.
In split-cycle engines, the intake stroke follows the compression stroke, whereas, in conventional engines, the intake stroke follows the exhaust stroke. Accordingly, in a split-cycle engine, a small amount of compressed high pressure air is always trapped in the compression cylinder when the compression piston reaches its top dead center position. Because this compressed air must be re-expanded during the intake stroke before a fresh charge of air can be drawn in, the compression cylinder of a split-cycle engine must be sized to include the volume of re-expanded trapped air in addition to the volume of a fresh charge of air. This tends to increase the size and reduce the power density of a split-cycle engine relative to a conventional engine with the same intake charge.
Both split-cycle engines and conventional engines may have their intake pressures boosted, e.g. through turbocharging, supercharging or the like, to increase the power density and decrease the overall size of the engine. The greater the boost (i.e., increase in pressure above one atmosphere pressure absolute) over a naturally aspirated engine, the greater the power density and the more an engine may be downsized.
Problematically, the amount of boost that can be provided to the intake charge of either a conventional or split-cycle gasoline engine is limited by the point at which the fuel/air mixture is forced into premature combustion (i.e., knocking) by excessive pressures and temperatures developed within the engine during the compression stroke. It is well known that in a conventional gasoline engine with a single stage turbocharger, the boost pressure is limited to approximately 1.5 to 1.7 bar absolute before knocking will occur. Higher boost pressures are potentially attainable, but require expensive multistage boosting and intercooling systems in prior art conventional engines.
Accordingly, there is a need to increase the resistance to knock for engines generally and for split-cycle engines in particular. More specifically, there is a need to increase the knock resistance of split-cycle engines such that they may have their intake charge boosted to pressures of 1.7 bar absolute or greater.